Fiber optic pressure sensitive optical switch and apparatus incorporating same

ABSTRACT

An optical switch operatively sensitive to a fluid pressure differential includes a house having an open end, a fiber optic input line, a fiber optic output line, and a fluid-proof diaphragm sealingly engaged with the housing. The diaphragm is formed from a thin elastic material resiliently stretchable between first and second switching states and supports an optical barrier within the housing for blocking the propagation of an optical signal between the fiber optic lines when the diaphragm is in one switching state and for permitting the propagation of the signal between the fiber optic lines when the diaphragm is in the other one of the switching states. For a given housing and a given diaphragm, different optical barriers may be employed to achieve different switching thresholds. As well differing optical barriers may be used to make the switch either a normally &#34;on&#34; switch or a normally &#34;off&#34; switch. Apparatus for monitoring the level of a liquid may include a plurality of such switches.

FIELD OF THE INVENTION

The present invention relates to pressure sensitive optical switches andto apparatus comprising a plurality of such switches that isparticularly suitable for monitoring the level of a liquid.

BACKGROUND TO THE INVENTION

The use of optical fibers produced from glass, plastic or syntheticfused silica, often called silica or quartz fiber, is well known. Onearea where fiber optic technology has been developed to play asignificant role is for the detection or measurement of liquid levels.In this regard, and as is indicated in U.S. Pat. No. 5,072,617 grantedto Weiss on Dec. 17, 1991, the detection of liquid level is one of manyfunctions for which fiber optic sensors may be more suitable than theirelectrical counterparts, especially in noisy or hostile environments.

In some applications, it may be highly desirable to provide a continuousindication of pressure or liquid level. However, in many applications,it may be quite sufficient to detect pressure or liquid levels atdiscrete levels. By way of example, if it is desired to monitor thelevel of a crude oil or other liquid within a hold of a ship, then itmay be sufficient to detect the fullness of the hold within incrementsof 25%. Continuous measurements together with the relatively complexdesign considerations and instrumentation required to effect suchmeasurements over broad pressure ranges may be considered unnecessaryand impractical.

The prior art includes a variety of fiber optic devices that aredesigned to react to external fluid pressures applied to the device,whether as the result of a rising liquid level or otherwise. One classof such devices relies upon the use of a flexible membrane or diaphragmthat resiliently stretches in response to external pressure and, in sodoing, modifies the transmission of optical signals being carried to anfrom the device by fiber optic lines. Advantageously, the operation ofdevices that incorporate a pressure sensitive diaphragm generally doesnot depend upon the transmission of optical signals through the fluidmedium under scrutiny. Accordingly, the operation is independent of theoptical properties of the fluid.

One example of a pressure sensitive device that utilizes a flexiblediaphragm is disclosed in U.S. Pat. No. 5,408,546 granted to Slaker etal. on Apr. 18, 1995. In this case, an optical signal is transmittedwithin a cylindrical housing or probe from one end and reflected backfrom an opposite end. The reflected signal is then detected at the firstend. An opening in the wall of the housing is covered by a flexiblemembrane or diaphragm that normally appears to bulge into the housingand into the path of a transmitted signal. As disclosed, the device isapparently designed and used for continuous pressure measurements whichare negative with respect to an ambient pressure and it is not clear howit might be easily adapted to sense or detect increasing positivepressure in a switching mode. In any case, for a given housing and agiven diaphragm, the operating characteristics would be largely fixed.It does not appear that such characteristics could be altered except byusing a diaphragm having different resilient characteristics and/or bymodifying the size of the diaphragm in relation to the housing. Further,if the device could be adapted to sense or detect positive pressures ina switching mode, the device would be a normally "on" switch, and it isnot apparent how it might be modified to operate as a normally "off"switch if a normally "off" characteristic was desired.

The patent to Weiss, supra, is another example of a pressure sensitivedevice that utilizes a flexible diaphragm. Weiss discloses a fiber-opticliquid level sensor that measures the height of a column of liquidthrough the hydrostatic pressure produced by the column on thediaphragm. As in the case of Shaker et al., the design of Weiss isintended for continuous measurements. But, unlike Slaker et al. wherethe diaphragm bulges into the path of a transmitted optical signal, thediaphragm of Weiss is used as a reflector that always stands completelyin the path of a transmitted signal. The angle of reflection variesdepending upon the degree of bulge thereby varying the signal intensityas seen by an associated signal detector. Such an arrangement requires areflective diaphragm and the careful maintenance of optical alignmentbetween input and output fiber optic lines and the diaphragm. Further,as in the case of Slaker et al., the operating characteristics of thedevice would be largely fixed for a given diaphragm.

A primary object of the present invention is to provide a new andimproved pressure sensitive optical device of the class that utilizes aflexible diaphragm and that is particularly suitable for operation as anoptical switch having on and off switching states.

A further object of the present invention is to provide a pressuresensitive optical switch where, for a given diaphragm, differingswitching thresholds can be achieved with minimal design variation.

A still further object of the present invention is to provide a pressuresensitive optical switch where, for a given diaphragm, the switch may beoperated as a normally "on" switch or, with minimal design variation, asa normally "off" switch.

Yet another object of the present invention is to provide new andimproved apparatus utilizing a plurality of such switches for monitoringthe level of a liquid.

SUMMARY OF THE INVENTION

In a broad aspect of the present invention, there is provided an opticalswitch operatively sensitive to a fluid pressure differential, theswitch comprising a housing having an open end, a fiber optic inputline, a fiber optic output line, and a fluid-proof diaphragm sealinglyengaged with the housing across the open end to define a region insidethe housing and a region outside the housing. The fiber optic input lineincludes an input end for receiving an optical input signal from anoptical source outside the housing, and an output end in the insideregion for providing the optical input signal as an optical outputsignal propagating within the inside region. The fiber optic output lineincludes an input end in the inside region in optical alignment with theoutput end of the input line for receiving the propagating signal, andan output end for transmitting the propagating signal, when so received,as an optical output signal from the switch.

The diaphragm is formed from a thin elastic material resilientlystretchable between a first switching state which subsists when fluidpressure in the outside region equals fluid pressure in the insideregion and a second switching state which subsists when fluid pressurein the outside region exceeds fluid pressure in the inside region by apredetermined amount. An optical barrier supported by the diaphragmextends within the inside region for blocking the propagation of anoptical signal from the output end of the input line to the input end ofthe output line when the diaphragm is in one of the switching states andfor permitting the propagation of the optical signal from the output endof the input line to the input end of the output line when the diaphragmis in the other one of the switching states. For a given housing and agiven diaphragm, different optical barriers may be employed to achievedifferent switching thresholds.

Preferably, the optical barrier is centrally positioned on thediaphragm. It will then move a maximum distance as the diaphragmstretches between the first and second switching states. Also, the openend of the housing preferably has a circular perimeter. The switch willthen have a maximum sensitivity to pressure changes because stretchingforces on the diaphragm will be distributed over a maximum surface area.

As will become more apparent hereinafter, the optical barrier may beconstructed such that the switch operates as a normally "on" switch oras a normally "off" switch. In the case of a normally "on" switch, theoptical barrier permits propagation of an optical signal when thediaphragm is in its first switching state and blocks propagation of theoptical signal when the diaphragm is in its second switching state.Conversely, in the case of a normally "off" switch, the optical barrierpermits propagation of an optical signal when the diaphragm is in itssecond switching state and blocks propagation of the optical signal whenthe diaphragm is in its first switching state.

In a preferred embodiment, an optical switch in accordance with thepresent invention further includes a vent for reaching from the regioninside the housing to a region of ambient pressure distant from thehousing. Such a vent will serve to maintain the inside region at ambientpressure regardless of the pressure immediately outside the housing.Moreover, such a vent advantageously enables the sealed integrity of anassembled switch to be readily checked.

In another aspect of the present invention there is provided apparatusfor monitoring the level of a liquid, such apparatus comprising aplurality of optical switches as described above and a support forholding the switches in spaced succession. The diaphragm of each of theswitches is stretchable between the first and second switching states ofthe switch in response to the rise of the liquid from a level below theswitch to a level at which the switch is immersed to a predetermineddepth in the liquid.

Typically, it is contemplated that such apparatus may be used to monitorthe level of liquid in a tank or other container at (for example, crudeoil in a hold of a ship) at discrete increments such as 1/4 full, 1/2full, 3/4 full or completely full. If the tank has a uniform horizontalcross-section from top to bottom (e.g. an upright cylindrical tank) thenthe space between successive ones of the switches will be equidistant toenable such measurements. On the other hand, if the tank has anon-uniform horizontal cross-section from top to bottom (e.g. acylindrical tank lying on its side), then the space between successiveones of the switches will be correspondingly non-uniform to enable suchmeasurements.

The foregoing and other features and advantages of the present inventionwill now be described with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a normally "on" optical switch in accordancewith the present invention.

FIG. 2 is a side elevation view taken along plane 2--2 of FIG. 1.

FIG. 3 is a partially exploded and partially sectioned view of theoptical switch shown in FIG. 1, the switch being shown in an "on"switching state. The section is taken along cutting plane 3--3 of FIG.1.

FIG. 4 is a partially exploded and partially sectioned view of theoptical switch shown in FIG. 1, the switch being shown in an "off"switching state.

FIG. 5 is a partially exploded and partially sectioned view of anormally "off" optical switch in accordance with the present invention,the switch being shown in an "off" switching state.

FIG. 6 is a partially exploded and partially sectioned view of theoptical switch shown in FIG. 5, the switch being shown in an "on"switching state.

FIG. 7 is a perspective view of the optical barrier forming part of theoptical switch shown in FIGS. 5 and 6.

FIG. 8 is a partially diagrammatic and partially schematic view ofapparatus in accordance with the present invention for monitoring thelevel of a liquid.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the drawing, FIGS. 1 to 4 illustrate one embodiment of an opticalswitch in accordance with the present invention; FIGS. 5 to 7 illustrateanother. Both embodiments include a housing generally designated 20, adiaphragm 60, a fiber optic input line 75, a fiber optic output line 85,and a vent line 95. The only difference between the two embodiments isthe structure of the optical barrier forming part of the switch. In thecase of the embodiment illustrated in FIGS. 1 to 4, the switch includesan optical barrier 63 described below in more detail. In the case of theembodiment illustrated in FIGS. 5 to 7, the switch includes an opticalbarrier generally designated 65 and described below in more detail.

Common Features of Both Embodiments

Housing 20 is constructed from a cap 21 and a base sleeve 24, each ofwhich is press fitted over a cylindrical pipe section or nipple 32. Theintegrity of the press fits may be enhanced with a sealing glue (notshown) to better ensure that fluids to which the switch is exposedduring use cannot leak between the walls of the housing. Of course, theglue should be compatible with the fluids.

Sleeve 24 includes an inwardly extending annular flange 25 and a pair ofdiametrically opposed outwardly extending mounts 26, 27. Inner wall 23of flange 25 defines a circular open end of housing 20.

Sleeve 24 further includes bores 28, 29 which are rotationally alignedwith corresponding bores 33, 34 in pipe nipple 32. Together, such boresprovide diametrically opposed openings through the wall of housing 20,such openings being sized to receive associated fiber optic lines 75,85. Cap 21 includes an opening or bore 22 which is sized to receivetubular vent line 95. Lines 75, 85 and 95 are all secured in theirrespective bores utilizing glue or other suitable means to prevent theleakage of fluids between the lines and the bores.

Housing 20 can be made from various suitable materials including metalsand plastics. Provided that the environment to which the housing isexposed is not hostile, a polypropylene plastic may be used.

Diaphragm 60 extends across the open end of housing 20 defined by innerwall 23 and is formed from a resilient fluid-proof material sealinglyengaged with housing 20 by annular extensions 61, 62 of the material. Asbest seen in FIGS. 3 to 6, diaphragm 60 serves to define a region R_(i)inside housing 20 and a region R.sub.φ outside the housing.

Annular extension 61 of the diaphragm material is secured between flange25 of sleeve 24 and the bottom of pipe nipple 32. Annular extension 62is secured between the outer cylindrical wall surface of sleeve 24 andthe inner cylindrical wall surface of pipe nipple 32. It will beunderstood that the perimeter of diaphragm 60 (viz. the line where itpasses under pipe nipple 32 and merges with annular extension 61) issubstantially circular.

Fiber optic input line 75 includes an input end 76 for receiving anoptical input signal from an optical source (not shown) outside housing20, and an output end 77 in region R_(i) for providing the input signalas an optical output signal propagating within region R_(i). Fiber opticoutput line 85 includes an input end 86 in region R_(i) in opticalalignment with output end 77 for receiving the propagating signal fromline 75 and an output end 87 for transmitting the signal, when soreceived, as an optical output signal from the switch.

Vent line 95, only a short part of which is shown, is an elongatedflexible plastic tube which, when connected to housing 20 through bore22 provides a vent for reaching from region R_(i) inside housing 20 to aregion of ambient pressure (not depicted) distant from the housing. Thepressure within housing 20 is thereby maintained at ambient pressureregardless of the pressure in outside region R.sub.φ immediately outsidehousing. In effect, the switch is rendered insensitive to changes inambient pressure--a result which will be desirable unless expectedchanges in ambient pressure are insignificant, or significant changes inambient pressure are the very thing that one wishes to sense. Further,and as noted above, such a vent enables the sealed integrity of theswitch to be readily checked. More particularly, if housing 20 iscompletely immersed in water or some other liquid and air is then forceddown the vent, any resulting bubbles from the switch will indicate thatthe housing is not properly sealed.

As in the case of housing 20, diaphragm 60 may be made from varioussuitable materials. One material which is suitable for variousenvironments is elastic latex. However, it should be noted that latexwould not be suitable for all environments. For example, in anenvironment such as diesel fuel an elastic neoprene or nitrile rubberwould be preferred.

When the switch depicted by FIGS. 1 to 4 or the switch depicted by FIGS.5 to 7 is initially assembled, the pressure inside housing 20 (regionR_(i)) and the pressure outside the housing (region R.sub.φ) will beequal. So long as these pressures remain equal, the state of diaphragm60 will be basically as depicted in FIGS. 3 and 5; viz. extending in aflat plane across the open end of the housing. Subsequently, if thepressure in outside region R.sub.φ increased, then diaphragm 60 will beforced to stretch or bulge upwardly as depicted in FIGS. 4 and 6. Thevolume of region R_(i) will reduce until the spring return force of thediaphragm is sufficient to balance the increased pressure.

Embodiment Shown in FIGS. 1 to 4

As noted above, the unique feature of the embodiment shown in FIGS. 1 to4 compared to the embodiment shown in FIGS. 5 to 7 is optical barrier63. Barrier 63 has an optically opaque cylindrical block constructioncentrally positioned on and extending upwardly from diaphragm 60 withinregion R₁. Thus, when diaphragm 60 bulges upwardly as shown in FIG. 4,then barrier 63 is lifted by a corresponding maximum distance upwardly.In other words, the movement of barrier 63 is maximally sensitive tochanges in pressure within region R.sub.φ, relative to region R_(i).

If diaphragm 60 is made from a latex material as noted above, thenbarrier 63 advantageously may be made from the same or similar materialheat welded to the diaphragm to form a single piece.

When barrier 63 is in the position or switching state shown in FIG. 3,there is a clear line of sight 80 from output end 77 of fiber opticinput line 75 to input end 86 of fiber optic output line 85. Incontrast, when barrier 63 is in the position shown or switching statesshown in FIG. 4, the line of sight 80 is blocked.

In the operation of the embodiment shown in FIGS. 1 to 4, an opticalinput signal received at input end 76 of line 75 is transmitted alongline 70 to output end 77. Provided that the line of sight 80 is notblocked by optical barrier 63, then the input signal will propagatethrough region R_(i) from output end 77 of line 70 to input end 86 ofline 85. Line 85 transmits the received input signal to output end 87 ofline 85.

Normally, the line of sight 80 is not blocked by barrier 63. That is,the normal state of diaphragm 60 is the initial state or conditiondepicted in FIG. 3 where the diaphragm extends in a horizontal planethereby positioning barrier 63 below line of sight 80. Since an inputsignal will be transmitted through the switch when diaphragm 60 is inthis state, the switch may be considered as a normally "on" switch.

Subsequently, if the pressure in outside region R_(i) increases relativeto the pressure in inside region R_(i) by a sufficient amount asrepresented by arrows Δp in FIG. 4, then optical barrier 63 will beelevated to block the line of sight 80. Then, any input signal receivedat input end 76 of line 75 will fail to appear at output end 87 of line85. In this condition, the switch is in an "off" state.

Strictly, it will be noted that as the switch alters from an "off" stateto an "on" state, or vice-versa, there will be a transition region wherethe switch is neither fully "off" nor fully "on". In this region,barrier 63 impedes only a part of the line of sight 80 therebypermitting a portion of any input signal present on line 75 to propagateto line 85. However, as a practical matter, a threshold can be definedwhere the switch is considered by definition to be "or" if the intensityof an optical signal present at output end 87 of line 85 is above thethreshold--and otherwise is considered by definition to be "off".

It will be understood that the pressure increase Δp required to switchfrom an "on" state to an "off" state will be a function of the height ofoptical barrier 63 above diaphragm 60. A switch having an opticalbarrier with less height than barrier 63 would require a greaterpressure increase to switch from "on" to "off". Conversely, a switchhaving a barrier with more height than optical barrier 63 would requirea lesser pressure increase to switch from "on" to "off". For a givenhousing 20 and diaphragm 60, the switching threshold can be convenientlychanged merely by substituting one barrier for another.

Embodiment Shown in FIGS. 5 to 7

As noted above, the unique feature of the embodiment shown in FIGS. 5 to7 compared to the embodiment shown in FIGS. 1 to 4 is optical barrier65. Like barrier 63, barrier 65 has an optically opaque construction andadvantageously may be made from material the same as or similar todiaphragm 60. It is centrally supported by diaphragm 60 and extendsupwardly within region R_(i). However, unlike optical barrier 63,optical barrier 65 does not have a simple cylindrical blockconstruction. As best seen in FIG. 7, barrier 65 has an overallcylindrical construction, but includes a rectangular opening or aperture66 which passes through the barrier below solid upper portion 67 of thebarrier.

As best seen in FIG. 5, the height of barrier 65 is arranged such thatupper portion 67 normally blocks line of sight 80. Necessarily, theswitch is in an "off" state in this condition. However, if the pressurein outside region R_(i) increases by a sufficient amount over thepressure in inside region R_(i) (as represented by arrows Δp in FIG. 6),then optical barrier 65 is elevated is shown in FIG. 6 to a positionwhere there is a clear line of sight 80 through opening 66. In thiscondition, the switch is in an "on" state. In this position, opening 66provides a window through which an optical signal may propagate fromoutput end 77 of line 75 to input end 86 of line 85.

In effect, the use of optical barrier 65 instead of a barrier such asoptical barrier 63 serves to make the switch shown in FIGS. 5 to 7 anormally "off" switch rather than a normally "on" switch. The pressureincrease Ap required to switch from an "off" state to an "on" state is afunction of the height of aperture 66 above diaphragm 60. A switchhaving an aperture with less height than aperture 66 would require agreater pressure increase to switch from "off" to "on". Similarly, aswitch having a barrier with more height than aperture 66 would requirea lesser pressure increase to switch from "on" to "off". Thus, for agiven housing 20 and diaphragm 60, and as in the case of the embodimentshown in FIGS. 1 to 4, the switching threshold can be convenientlychanged merely by substituting one barrier for another. As well, it willnow be understood that for a given housing 20 and diaphragm 60 thecharacter of the switch as a normally "on" switch or as a normally "off"switch can be conveniently altered merely by substituting a barrier suchas barrier 65 for a barrier such as barrier 63 (or vice-versa).

Elimination of Vent

While an optical switch in accordance with the present invention mayadvantageously include a vent, a vent is not considered essential forall situations. For example, a switch similar in construction to theembodiment shown in FIGS. 1 to 4, or to the embodiment shown in FIGS. 5to 7, could be constructed without a vent. Such a result would beachieved by the elimination of vent line 95 and bore 22 in cap 21. Inthis situation, housing 20 would be completely sealed from the time ofassembly. Subsequently, if the pressure in outside region R.sub.φincreased, then diaphragm 60 would be forced to stretch or bulgeupwardly generally as described above. However the upward movement ofbarrier 63 or barrier 65, as the case may be, would be less than thecorresponding movement that would occur if a vent was present. In theabsence of a vent, the movement would be opposed not only by the springreturn force of diaphragm 60 but also by increasing pressure resultingfrom the volume reduction of in region R_(i) which accompanies suchmovement.

Of course, in the absence of a vent, then the sealed integrity of theswitch could not be checked in the manner discussed above. Further,other disadvantages may arise. For example, in the case of an embodimentsimilar to that shown in FIGS. 1 to 4, or to that shown in FIGS. 5 to 7,but without a vent, a temporary hole or opening in cap 21 may berequired during the process of assembly. Otherwise, as cap 21 is fittedover pipe nipple 32 with diaphragm 60 already in place, or vice-versa,there may be a tendency for pressure to build up in region R_(i)sufficient to bulge the diaphragm outwardly or downwardly from theneutral positions shown in FIGS. 3 and 5. As well, a change in ambientpressure after the time of assembly would produce a correspondingoutward or inward bulge of diaphragm 60 from its neutral position. Theresult would be to alter the apparent switching threshold of the switch.If any expected changes in ambient pressure were insignificant comparedto the increase in pressure required to activate the switch, then thealteration may be considered immaterial. Otherwise, unless the object ofthe switch was to sense significant changes in ambient pressure, thealteration may compromise the desired accuracy of measurements made withthe switch.

Liquid Level Monitor

Referring now to FIG. 8, there is shown an apparatus or probe generallydesignated 100 which includes a plurality of optical switches SW₁, SW₂,SW₃ in accordance with the present for monitoring the level of a liquid.All such switches may be normally "on" switches as described above inrelation to FIGS. 1 to 4 or, alternately, may be normally "off" switchesas described above in relation to FIGS. 5 to 7. Depending upon the case,each switch operates in the manner described above. For example, if allswitches are normally "on" switches, then they will progressively turn"off" as the liquid level rises from a level below the bottom of switchSW₁ to a level above the bottom of switch SW₃. Conversely, if allswitches are normally "off" switches, then they will progressively turn"on" as the liquid level rises from a level below the bottom of switchSW₁ to a level above the bottom of switch SW₃. As depicted in FIG. 8,liquid is indicated at a level 200 which is sufficient to activate allswitches except switch SW₃ either "off" or "on", as the case may be.Switch SW₃ is not activated in FIG. 8 because it lies above level 200.

As can be seen, switches SW₁, SW₂, SW₃ are held in spaced succession bya support or framework comprising threaded tie rods 110, 112 which arejoined across their tops by a bracket 120, and which extend through andare coupled with opposed mounts 26, 27 of each switch. Bracket 120includes an eyebolt 125 to facilitate securing the support in a desiredposition. If desired, a similar bracket with or without a similareyebolt could be provided at the lower end of the support below thebottom-most switch. If needed, such a bracket could be used to provideadded strength and stability by relieving undue stress on the switchesthemselves. Further, such a bracket would serve to shield thebottom-most switch from foreign objects that might otherwise strike itsdiaphragm (not shown in FIG. 8) from below.

Each successive switch SW₁, SW₂, SW₃ is spaced from its immediatelypreceding switch by an equal distance "d". Further, each successiveswitch includes a fiber optic input line I₁, I₂, or I₃, as the case maybe, a fiber optic output line .O slashed.₁, .O slashed.₂, or .Oslashed.₃, as the case may be, and a vent line comprising an individualline segment V₁, V₂, or V₃, as the case may be, and a common linesegment V₀. Common line segment V₀ terminates at its upper end 150 in aregion R_(a) of ambient pressure. Upper end 150 is directed downwardlyto obstruct the flow of any liquid through the vent lines.

Each fiber optic input line I₁, I₂, I₃ is like input line 75 discussedabove and receives an optical input signal s₀ from an optical source(not shown). Signal s₀ is then routed along cable 130 to its associatedswitch SW₁, SW₂, or SW₃, as the case may be. Each fiber optic outputline .O slashed.₁, .O slashed.₂, .O slashed.₃ is like output line 85discussed above and transmits any signal that it receives within itsassociated switch housing as an optical output signal which is thenrouted along cable 140 where it ultimately appears as an output signals₁, s₂, or s₃, as the case may be.

Of course, it will be understood that any given switch SW₁, SW₂, SW₃will not activate (viz. turn "off" or "on" depending upon whether it isa normally "on" or normally "off" switch) until it becomes immersed to asufficient depth in the liquid. If the switch is immersed only to arelatively shallow depth, then the added fluid or liquid pressure on thediaphragm of the switch (viz. like diaphragm 60 as described above) willbe insufficient to elevate the optical barrier of the switch by anamount sufficient to produce the desired switching action. Thus if thediaphragm of a given switch is meant to sense or monitor a particularlevel of liquid, then the switch necessarily must be positioned at apredetermined distance below that level. The required distance willdepend upon the sensitivity of the switch and the density of the liquidbut can be readily determined or calibrated for a given design of switchmerely by testing the switch at various depths in a sample of theliquid.

In operation, and apart from transitional states where optical signalsbetween lines I₁, I₂, I₃ and lines .O slashed.₁, .O slashed.₂, or .Oslashed.₃ are only partially blocked, the magnitude of output signalss₁, s₂, s₃, will be either 0 or s₀ depending upon whether the associatedswitch SW₁, SW₂, SW₃ is in an "off" state or an "on" state. Variousmeans well known to those skilled in the art may be used to transmitsignal s₀. Likewise, various well known means may be used to receivesignals s₁, s₂, s₃ and to display or otherwise annunciate their status.In a most basic system, and provided that the fiber diameter of lines .Oslashed.₁, .O slashed.₂, or .O slashed.₃ is sufficiently large, one maysimply observe, visually, whether light is or is not present at theoutput of the lines as a measure of whether one or more of theassociated switches have been activated by the level of a liquid beingmonitored. For example, a 1 mm diameter acrylic fiber line is sufficientfor visual observation.

Since switches SW₁, SW₂, SW₃ are vented to region R_(a) of ambientpressure, it will be appreciated that their operation will be unaffectedby any changes in ambient pressure. The pressure inside each switch willalways correspond to ambient pressure.

In any specific application, it will be understood that the total numberof switches may be more or less thin three switches shown in FIG. 8.Likewise, the length of tie rods 110, 112 and the relative scale ofdistance "d" between switches may be longer or shorter than thatdepicted in FIG. 8. Further, the distance "d" between successiveswitches need not be the same. All of this will depend upon the maximumdepth of liquid to be measured and the desired measurement resolution atlesser depths. For example, if it was desired to monitor the liquidlevel within a tank or other liquid container (not shown) having auniform horizontal cross section, and to do so at 25% incrementsrepresenting 1/4 full, 1/2 full, 3/4 full and completely full, then fourswitches would be required at intervals of h/4 where level h representsa full tank. However, if the tank or container had a non-uniformhorizontal cross-section, then uniformly spaced switches could obviouslynot represent equal increments of fullness. To measure equal incrementsof fullness, the spacing between successive switches would have to bemade correspondingly non-uniform. With switches mounted on tie rods inthe manner shown in FIG. 8, the adjustments necessary to achieve arequired non-uniform spacing can be readily made along the length of thetie rods.

It should be noted that switches SW₁, SW₂, SW₃ may be supported by meansother than the particular means shown in FIG. 8. Further, with respectto the embodiment shown in FIG. 8, it should be noted that only one ofthe two tie rods 110, 111 shown may be considered to suffice for somecases. In such cases it is contemplated that an eyebolt like eyebolt125, or other suitable securing means, would be located at or madeintegral with the top of the one tie rod used. An advantage of only tierod is that position of the switches can be adjusted more easily. Adisadvantage is that the switches are more exposed to damage fromforeign objects.

A variety of modifications, changes and variations to the invention arepossible within the spirit and scope of the following claims. Theinvention should not be considered as restricted to the specificembodiments that have been described and illustrated with reference tothe drawings.

I claim:
 1. An optical switch operatively sensitive to a fluid pressuredifferential, said switch comprising:(a) a housing having an open end;(b) a fluid-proof diaphragm sealingly engaged with said housing acrosssaid open end to define a region inside said housing and a regionoutside said housing, said diaphragm being formed from a thin elasticmaterial resiliently stretchable between a first switching state whichsubsists when fluid pressure in said outside region equals fluidpressure in said inside region and a second switching state whichsubsists when fluid pressure in said outside region exceeds fluidpressure in said inside region by a predetermined amount; (c) a fiberoptic input line having:(i) an input end for receiving an optical inputsignal from an optical source outside said housing; and, (ii) an outputend in said inside region for providing said optical input signal as anoptical output signal propagating within said inside region; (d) a fiberoptic output line having:(i) an input end in said inside region inoptical alignment with said output end of said input line for receivingsaid propagating signal; and, (ii) an output end for transmitting saidpropagating signal, when so received, as in optical output signal fromsaid switch; and, (e) an optical barrier supported by said diaphragm andextending within said inside region for blocking the propagation of saidoptical signal from said output end of said input line to said input endof said output line when said diaphragm is in one of said switchingstates and for permitting the propagation of said optical signal fromsaid output end of said input line to said input end of said output linewhen said diaphragm is in the other one of said switching states.
 2. Anoptical switch as described in claim 1, wherein said optical barrier iscentrally positioned on said diaphragm.
 3. An optical switch asdescribed in claim 2, wherein said optical barrier permits propagationof said optical signal when said diaphragm is in said first switchingstate and blocks propagation of said optical signal when said diaphragmis in said second switching state.
 4. An optical switch as described inclaim 2, wherein said optical barrier permits propagation of saidoptical signal when said diaphragm is in said second switching state andblocks propagation of said optical signal when said diaphragm is in saidfirst switching state.
 5. An optical switch as described in claim 2,wherein said open end has a circular perimeter.
 6. An optical switch asdescribed in claim 5, wherein said optical barrier permits propagationof said optical signal when said diaphragm is in said first switchingstate and blocks propagation of said optical signal when said diaphragmis in said second switching state.
 7. An optical switch as described inclaim 5, wherein said optical barrier permits propagation of saidoptical signal when said diaphragm is in said second switching state andblocks propagation of said optical signal when said diaphragm is in saidfirst switching state.
 8. Apparatus for monitoring the level of aliquid, said apparatus comprising:(a) a plurality of optical switches asdescribed in claim 1, the said diaphragm of each given one of saidswitches being stretchable between said first and second switchingstates in response to the rise of said liquid from a level below thegiven switch to a level at which the given switch is immersed to apredetermined depth in said liquid; and, a support for holding saidswitches in spaced succession.
 9. Apparatus as described in claim 8,wherein successive ones of said switches are spaced equidistant fromeach other.
 10. Apparatus as described in claim 9, wherein said opticalbarrier of each given one of said switches is centrally positioned onsaid diaphragm of said given switch.
 11. Apparatus as described in claim10, wherein said optical barrier of each given one of said switchespermits propagation of said optical signal when said diaphragm of saidgiven switch is in said first switching state and blocks propagation ofsaid optical signal when said diaphragm of said given switch is in saidsecond switching state.
 12. Apparatus as described in claim 10, whereinsaid optical barrier of each given one of said switches permitspropagation of said optical signal when said diaphragm of said givenswitch is in said second switching state and blocks propagation of saidoptical signal when said diaphragm of said given switch is in said firstswitching state.
 13. Apparatus as described in claim 10, wherein saidopen end of each of said switches has a circular perimeter. 14.Apparatus as described in claim 13, wherein said optical barrier of eachgiven one of said switches permits propagation of said optical signalwhen said diaphragm of said given switch is in said first switchingstate and blocks propagation of said optical signal when said diaphragmof said given switch is in said second switching state.
 15. Apparatus asdescribed in claim 13, wherein said optical barrier of each given one ofsaid switches permit, propagation of said optical signal when saiddiaphragm of said given switch is in said second switching state andblocks propagation of said optical signal when said diaphragm of saidgiven switch is in said first switching state.
 16. Apparatus asdescribed in claim 8, wherein said support comprises a tie rod, saidswitches being mounted at selected positions along the length of saidtie rod.
 17. Apparatus as described in claim 8, wherein said supportcomprises a tie rod, said switches being mounted at adjustable positionsalong the length of said tie rod.
 18. Apparatus as described in claim 8,wherein each of said switches includes an associated vent line forproviding air flow communication between said region inside said housingof the associated switch and an external region of ambient pressurewhile any one or more of said switches are immersed in a fluid.
 19. Anoptical switch as described in claim 1, further including a vent linefor providing airflow communication between said region inside saidhousing and an external region of ambient pressure while said switch isimmersed in a fluid.